In the graphic arts industry, which encompasses the printing, lithographing, duplicating, publishing, advertising, commercial arts, and photoengraving fields, registration of film images is required before proceeding to the mechanics of producing the finished plates which are ultimately used for printing. Specifically, the various components which are to make up the resulting printed product must first be photographed and the resulting photographic images must then be positioned and assembled together to form a composite film negative image which is used to produce, i.e. "burn", the printing plate. This assembly process is referred to in the art as the masking or "stripping" step and is typically performed on a light table.
The masking or "stripping" of film work is performed by strippers who assemble, with precision, the various film components used to produce the composite proofs and burn the printing plates. The assembly mounting of these components is tedious work due to the necessity to precisely align and position the individual components.
Production stripping almost invariably includes the use of a masking sheet, commonly referred to as a flat, in a color which blocks the ultraviolet light used to burn the printing plate. The masking sheet is used to align and maintain the relative position of the various portions of the composite image and to control the portions of the image appearing on the printing plate. Specifically, the masking sheet and film images are placed on a light table, with the masking sheet overlying the film images. Although masking sheets block ultraviolet light, they are typically made of a material which is permeable by visible light so that the underlying film images are visible through the masking sheet when they are placed on a light table. The underlying film images are manipulated and positioned to achieve the desired composite image, at which point they are affixed to the underside of the masking sheet by an adhesive material such as, for example, adhesive tape. The masking sheet is then selectively cut away to expose the portions of the composite image which are to appear on the printing plate. The entire assembly, i.e. the masking sheet and the affixed composite image, is then transferred to a plate making machine where it is used to burn the printing plate.
In the past, masking sheets have typically been formed of either paper, coated or uncoated, or a vinyl, known in the art as printer's vinyl. These sheets are produced in a color (either orange or red in vinyl, or orange, red or goldenrod in paper), which masks the range of ultraviolet light produced by the plate burner.
Paper masking sheets are advantageous because they are often provided with preprinted grids which the stripper can use to align the film images before burning the plates. Paper masking sheets do, however, suffer from several drawbacks which make them unsatisfactory in many applications. Paper masking sheets are particularly problematic because they are not durable. Specifically, masking sheets typically include a border which is outside the image area, i.e., an area equal to the usable space on the resulting printing plate. Registration holes may be formed in this border and used in conjunction with registration pins to maintain alignment of the masking sheet and film images both during the stripping and plate burning processes. However, when paper masking sheets are employed, the registration holes are easily torn or deformed, making alignment of the film images difficult and imprecise. Paper masking sheets are also unsuitable for accurately reproducing duplicate printing plates at a later date because the paper can be deformed by shrinking and/or expansion due to temperature and/or humidity changes.
Vinyl masking sheets overcome the above noted problems with paper masking sheets. However, to date, preruled vinyl masking sheets are not commercially available. As a result, strippers are forced to manually rule the vinyl, e.g., with a pencil and a ruler. These manually ruled grids are invariably inaccurate, time consuming to construct, and are often smudged by use or storage because of the difficulty of permanently marking vinyl from which masking sheets are made.
Hence, there is a need to provide a preruled printing masking sheet which is constructed from a material which overcomes the deficiencies associated with paper masking sheets.
Accordingly, an object of the present invention is to provide a preruled printing masking sheet which is durable and precise.
Another object of the present invention is to provide a printing masking sheet which can be reliably reused to produce printing plates over an extended period of time.
Still another object of the present invention is to provide a printing masking sheet which is economical and simple to manufacture.